It is desirable in the fabrication of composite materials, e.g. those of chipboard, fiberboard, particle board, wood laminates, plywood and the like to provide thermally activatable synthetic-resin precursors which are polymerized, polycondensed or condensed to form binders or bonding agents.
It is also known to use synthetic-resin precursors usually vinylic monomers in impregnating solutions for individual pieces of wood, assemblies of wood pieces or the like and to effect thermal polymerization, condensation or polycondensation in situ by heating the wood body impregnated with the monomeric solution. In the thermal polymerization of synthetic-resin precursors, especially in wood, the selection of the heat carrier plays an important role since this heat carrier is used for a number of purposes. For example, in the case of exothermic polymerization reactions, it may serve to bring the system to the threshold temperature at which the reaction is initiated. In the case of endothermic reactions, it may provide the necessary thermal energy for maintaining the reaction. Also in the case of exothermic reactions, it may serve as the agent for carrying away excess thermal energy.
Not all heat carriers are effective for all purposes as will be apparent hereinafter. Gaseous heat carriers have been found to effect heat exchange so slowly that the reactions /require many hours, a significant technological disadvantage.
To improve the heat transfer efficiency, it has already been proposed to effect thermal polymerization with impregnated wood using a liquid heat carrier. Even here, however, problems have been discerned with selection of the heat-transfer agent. For example, oil (see French Patent FR-PS 694,690) has the disadvantage that it mixes with the low-viscosity monomeric materials at temperatures of 60.degree. to 100.degree. C. or solubilizes the monomers wherever they are accessible to the oil through the pores of the workpiece.
Because of thermal expansion and contraction effects and density changes during the polymerization (the specific gravity of the monomer changes from about 0.91 g/cm.sup.3 to 1.15 g/cm.sup.3 in the transformation to the polymer), there is a reduction in the volume of the polymer wood and a penetration into the pores thereof formed during the shrinkage of the polymer. As a result, there are unpredictable, nonuniform, partly irreversible physical and chemical changes in the surfaces of the workpiece resulting in discoloration and inhomogeneities which may require a time-consuming and expensive surface treatment process, e.g. organic-solvent washing, or sanding and other abrading operations which result in high losses of the polymer wood.
The use of metal alloy melts (Japanese Patent JA-PS 711 48 39) have also been proposed but are disadvantageous at temperature ranges of 60.degree. and 100.degree. C. The metal alloys of tin, antimony, bismuth, lead and zinc are very expensive. Their vapors are toxic and require special measures for environmental protection.
At temperatures of 60.degree. C. to 100.degree. C., the viscosity of these metal alloys is relatively high and it is therefore energy-expensive to recirculate the medium to the satisfactory degree. Furthermore, when the impregnated wood substrate is introduced in a cold state into a bath of the alloy, a surface cooling of the bath is extremely inconvenient so that measures must be taken to prevent such phenomena.
Finally, in this connection, it should be noted that the immersion of the impregnated wood substrate in the hot melt results in the displacement of monomer components into the metal bath from the impregnated substrate which causes the pickup of impurities by the bath at the wood surface and a time-consuming and costly cleaning process.
It has already been proposed to use aqueous solutions as heat carriers (see German open application - Offenlegungsschrift--2 111 149). Such systems are always employed at the highest possible viscosity since it has been the assumption that only in this way it is possible to prevent the penetration of the heat carrier into the workpiece and the expulsion of the monomers therein.
The handling of such high-viscosity solutions, usually sodium alginate, water glass and glycerine, is difficult. The workpieces of wood or like materials must be introduced into the treating solution with relatively large spacings so that all surfaces will come into contact with the solution in spite of its high viscosity. Should a portion of the solution be vaporized, the resulting boilup gives rise to a foam development which is disadvantageous to the operating personnel. Finally, it is expensive and inconvenient to remove the residues of such viscous materials from the workpieces after polymerization has been effected to the desired degree.